1. Field of the Invention
The present invention relates to reflective adjustable optical deflectors employing elements with electro-optical effects (the Pockels effect). The invention also relates to optical devices, such as variable optical attenuators and optical switches, employing the reflective adjustable optical deflectors.
2. Description of the Related Art
The following (1) through (4) methods have been used as techniques for attenuating light.
(1) Shields disposed on optical paths are mechanically moved to change the transmittance.
(2) A material whose refractive index depends on temperature is inserted in the middle of a waveguide Mach-Zehnder interferometer, and either one of the optical paths, before or after the material, is heated or cooled to change the interference, the transmittance thereby being changed.
(3) A Faraday rotator is inserted between a polarizer and an analyzer, to vary the magnetism applied to the Faraday rotator, thereby rotating a polarization of transmission light. This causes a mismatch between the polarization of the beam and that of the analyzer, the transmittance thereby being changed.
(4) A portion of the cladding of an optical fiber is removed, and a material whose refractive index depends on temperature is filled thereinto. This changes the refractive index relationship between the core and the cladding, thereby inducing radiation, so that the transmittance is changed.
These methods have the following problem. Since the response speed in these methods is limited to a few hundred micro seconds at best, the methods cannot support a higher speed response which is required, for example, when adding/dropping signal channels and when dynamic crossconnect is performed in WDM systems.
In view of this, a technique has been reported in which is applied high-speed response technology that is originally for use in optical amplifiers or the like. A Mach-Zehnder interferometer is prepared using a waveguide which is formed by an element (for example, LiNbO3) with electro-optical effects, and voltage is applied to only either one of the optical paths, to induce attenuation. However, this technique still has problems of high insertion loss and high polarization-dependent loss. Further, when applying this technique to WDM transmission apparatus, variable optical attenuators equal in number to the multiplexed wavelengths are required. Considering numerous wavelengths transmitted in recent WDM transmission systems, the attenuators are inappropriate in size, resulting in increases in size of the WDM transmission apparatus.
With the foregoing problems in view, it is an object of the present invention to provide down-sized optical devices, such as variable optical attenuators and optical switches, which are high in response speed and low in insertion loss and polarization-dependent loss, by employing a reflective adjustable optical deflector in such devices.
Here, adjustable optical deflectors themselves, as disclosed in Japanese Patent Application Publication Number HEI 10-307307 and also in HEI 10-288798, are well known for use in scanners equipped in laser printers, laser scanning microscopes, and barcode readers, but are not suitable for use in wavelength-Division Multiplex (WDM) transmission apparatus.
This is because of the construction of the previous adjustable optical deflectors. More specifically, incoming light from the input optical system enters the deflector from its input side, and passes through the deflector, and goes out from its output side, which is disposed opposite the input side, and the transmitted light is then directly received by the output optical system. This construction makes it necessary to arrange the input optical system, the adjustable optical deflector, and the output optical system, in an almost straight line, thus causing downsizing limitation. Therefore, in cases where an increased number of channels are to be transmitted, this will cause increased apparatus size.